In recent years there has been growing use of spread spectrum systems due to the many advantages attainable by the use of such systems. One of the most desirable advantages is the ability of spread spectrum systems to operate well in the presence of interference, whether the interference is intentional or not. Therefore, spread spectrum systems are very attractive in situations where anti-jamming transmission and reception is desired.
One of the most common spread spectrum systems, especially for anti-jamming use, is the PN (pseudo-noise) sequence modulating type system. With the PN sequence system, a PN code is used in combination with digital information to modulate a carrier for transmission. Typically, some form of PSK (phase shift keying) is used for this modulation, although, of course, other modulation schemes could be used. The result of modulating a carrier with the PN code and the information is a broadband spread spectrum signal. This spread spectrum signal forces any jamming arrangement not using the same PN code to cover a wide frequency range at a much higher power level than the spread spectrum signal itself if interference is to be achieved.
When the spread spectrum signal is received, the same PN code is used to despread the signal. Once the PN code is removed, the information can readily be detected. However, to any other receiving system not knowing the PN code, the spread spectrum signal will appear essentially to be incomprehensible noise.
In prior art antenna array receivers for spread spectrum signals, the despreading is typically accomplished in the demodulator. As a result of this, each channel of the array requires an individual RF down-converter to convert the signal to a new frequency suitable for processing in the adaptive array processor (AAP) prior to passing to the modem. Use of a common receiver for down-conversion is not practical due to the precise phase and amplitude tracking required between signals over the extremely large bandwidth that would be necessary to frequency multiplex such broadband signals through a common receiver.
Therefore, the prior approach of utilizing down-converters in each channel accentuates one of the major problems of spread spectrum systems--namely complexity and high costs due to the large amount of hardware. Also, phase and amplitude tracking errors caused in these multiple down-converters combine with similar phase and amplitude tracking errors in the adaptive array processor to produce serious degradation of the null depth of the array system.